The present invention relates to a progressive power spectacle lens for presbyopia, and more particularly to a progressive power spectacle lens in which a power continuously changes between a distance portion and a near portion.
Conventionally, spectacle lenses are manufactured on request in accordance with specifications intrinsic to individual users. However, if a lens is manufactured from the beginning, various types of semi-finished lenses should be preliminarily prepared, and an appropriate one is selected and finished in accordance with the user's request. Typically, a semi-finished lens has a finished front surface, which has a progressive refraction power, and a back surface is formed to be a spherical surface of a toric surface depending on the user's request. In the specification, the front surface refers to a surface of the spectacle lens on an object side, and the back surface refers to a surface on an eye side.
The semi-finished lenses are divided, for example, into five groups having different ranges of vertex powers. Therefore, even though two spectacle lenses have different vertex power values, they might fall within the same group. In such a case, the same semi-finished lenses having the same front surfaces providing the same progressive refraction power are used, and the back surface is processed so as to meet the user's request. With this configuration, the number of moldings for the front surfaces providing the progressive power is reduced, thereby reducing a manufacturing cost.
Specifically, the vertex powers of the progressive power spectacle lenses (i.e., the spherical diopter SPH and the cylindrical diopter CYL) is divided into five value ranges, and only one type of semi-finished lens is prepared for each range. Therefore, even if two lenses having different vertex powers are required, if the vertex powers are included in the same range, the same type of the semi-finished lens is used for manufacturing the required lenses. Accordingly, the number of types of front surfaces providing the progressive power is limited, thereby decreasing the manufacturing cost.
FIGS. 23A-23C show ranges of vertex powers divided into groups I through V. FIG. 23A shows a minus range, FIG. 23B shows a plus range, and FIG. 23C shows a mixed range. A correspondence between the ranges and base curves (i.e., mean surface power at a distance reference point) of the semi-finished lenses are indicated in TABLE 1 below. For each range (i.e., group), a progressive power surface having a predetermined base curve is assigned. That is, for each range, a single semi-finished lens whose front surface has been processed to have a progressive power surface is assigned. It should be noted that the unit of measurement of the vertex power and base curve is D, diopter. TABLE 1 shows values when a refractive index is 1.60.
TABLE 1groupvertex powerbase curveI−10.00 to −6.25 0.50II−6.00 to −2.252.00III−2.00 to +1.004.00IV+1.25 to +3.005.00V+3.25 to +6.006.00
The above division is determined so that an optical performance of a lens falls within an allowable range for all the vertex powers within a range when the front surface is formed to have a common progressive power surface for the range.
For example, for group II which covers the spherical surface power range of −6.00 D to −2.25 D, the base curve of a progressive power surface is determined to be 2.00 D. Using this semi-finished lens, in order to obtain a lens whose SHP is −4.00 D and CYL is −0.00 D, the back surface of the lens is processed to have a spherical surface of −6.00 D. If a lens whose SHP is −6.00 D and CYL is −2.00 D is required, the back surface is formed to be a toric surface of −8.00 D and −10.00 D.
According to the above-described conventional method, for the central values of the ranges, the lenses may exhibit excellent optical characteristics. However, since the design freedom for the rear surface is limited, the optical performances may be deteriorated for the values close to the extremities of the ranges. As a result, according to the conventional method, it is impossible to obtain the lens having the optimum performance for all the vertex powers within the range.
For example, when addition power ADD is 2.00 D, in a group III which covers the SPH range of −2.00 D to +1.00 D, a progressive power surface with a base curve of 4.00 D is used for all the vertex powers within the group. FIG. 21A shows a surface power D1m (solid line) along a main meridian, on the progressive power surface, in a direction of the main meridian, and a surface power D1s (broken line) along the main meridian in a direction perpendicular to the main meridian.
FIGS. 21B to 21D show a power Pm (solid line) in the main meridian direction and a power Ps (broken line) in the direction perpendicular to the main meridian direction based on as-worn evaluation (i.e., evaluation based on measurement of the optical characteristics in the as-worn position) of the progressive power spectacle lens having the vertex powers of −2.00 D, 0.00 D and +1.00 D, respectively, which are obtained by processing the rear surfaces of the lenses having the common progressive surface. The transmission performance is evaluated by varying an object distance from infinitive to 300 mm, at an origin to a near portion.
As shown in FIG. 21C, for the vertex power of 0.0 D which is the center of the range, an excellent performance without astigmatism can be obtained. However, for the vertex powers of −2.00 D and +1.00 D at both extremities of the range, the astigmatism is generated at the distance portion and the lower area of the near portion as shown in FIGS. 21B and 21D. In FIGS. 21A-21D, the performance on the main meridian is indicated. However, such deviation in performance affects the performance of the entire lens surface, which will be further discussed with reference to FIGS. 22A-22D.
FIGS. 22A-22D show a distribution of the power of the lens shown in FIGS. 21A-21D on the lens surface using contour lines. FIG. 22A shows a surface performance of the progressive power surface which is a common front surface of lenses falling in group III. The left-hand side circle shows a surface astigmatism ASD of a surface defined by |D1m(Y)−D1s(Y)|, and the right-hand side circle shows a mean surface power APD defined by (D1m(Y)+D1s(Y))/2.
FIGS. 22B-22D shows performances of the progressive power spectacle lenses having the vertex diopter values of −2.00 D, 0.00 D and +1.00 D, respectively, using the transmission evaluation. In each figure, the left-hand side circle shows a distribution of astigmatism AS, and the right-hand side circle shows a distribution of average power AP.
It is understood by comparing the performances of FIGS. 22B and 22D with FIG. 22C, which corresponds to the center of the range, that for the extremity vertex powers −2.00 D and +1.00 D of the range, an area of distinct vision in which the astigmatism is less than a predetermined value is narrower both in the distance portion and near portion.